1. Field of the Invention
This invention relates to a boron nitride film and a process for preparing the same, and more particularly to a boron nitride film of excellent electrical insulating and heat conduction characteristics formed on a substrate in a manner to be preferentially oriented to a predetermined axis thereof which is suitable for use as an intermediate insulating layer in a three-dimensional semiconductor device to allow semiconductor layers each having a semiconductor element incorporated thereinto to be three-dimensionally laminated to accomplish the high density packaging of the semiconductor device or as an intermediate insulating layer in a semiconductor device for large power.
2. Description of the Prior Art
Recently, researches have been made in the three-dimensional packaging of a semiconductor device wherein semiconductor chips and/or layers are superposed to form one semiconductor element, with the high integration of a semiconductor element. However, it is required to solve various problems resulting from the structural features of a semiconductor in order to realize such three-dimensional packaging.
One of the problems is that it is difficult to form, on an intermediate insulating layer insulating interlayer a semiconductor device, a semiconductor layer having a crystallizability sufficient to incorporate a semiconductor element therein. When an insulating layer has been deposited on a semiconductor base plate of which the surface has roughness formed thereon due to a semiconductor element which has been already incorporated into the semiconductor base plate, it is impossible to grow a semiconductor layer of a good crystallizability on the insulating layer because the surface of the insulating layer exhibits an irregular state.
Another important problem is how to attain insulation between semiconductor layers of a semiconductor device and how to improve heat dissipation of the device. An electrical insulator is generally inferior in heat conduction characteristics, and electrical insulating properties and heat dissipation characteristics are contrary to each other. Thus, if both electrical insulating properties and heat dissipation characteristics of a semiconductor device are improved giving cheerful consideration to the selection of materials, the manufacturing of a semiconductor device and the like, the high integration and high density packaging of a semiconductor device can be significantly promoted.
The inventors proposed a beryllium oxide (BeO) and a method of preparing the same, and a semiconductor device using the BeO film in view of the foregoing problems of the prior art while taking notice of the fact that the use of a BeO film having excellent electric insulating properties and heat conduction characteristics as an insulating layer between semiconductor layers of a semiconductor device and the growth of at least one of the semiconductor layers on the BeO film allow the semiconductor layer to have a crystallizability sufficient to incorporate a semiconductor element thereinto. However, the prior art is often encountered with difficulty in the formation of such BeO film.
Now, attention is paid to boron nitride (hereinafter referred to as "BN") in view of the fact that it has the most excellent electrical insulating properties and heat conduction characteristics next to BeO. A BN film has been conventionally formed on a semiconductor layer according to a chemical vacuum deposition (CVD) process or a sputtering process. However, all the so-formed BN films are amorphous. Although, the film has a thermal conductivity substantially equal to stainless steel, it makes the epitaxial growth of a semiconductor layer thereon having a crystallizability sufficient to incorporate a semiconductor element thereinto substantially impossible.
The present invention has been made in view of the foregoing disadvantages of the prior art.
Accordingly, it is an object of the present invention to provide a BN film of excellent electrical insulating properties and heat conduction characteristics which is capable of exhibiting thermal conductivity about three times as much as the conventional amorphous BN film, and acting as an orientation film utilized as seeds for epitaxially growing a semiconductor layer thereon even if the BN film has been formed on an amorphous substrate or a substrate having roughness formed thereon.
It is another object of the present invention to provide a BN film which is capable of being used as an insulating layer between semiconductor layers of a three-dimensional semiconductor device to significantly promote the high integration and high density packaging of the device.
It is a further object of the present invention to provide a BN film which is capable of being used as an insulating layer between a heat dissipation plate and a semiconductor layer in a semiconductor device for large power to provide the device with excellent heat dissipation characteristics while keeping high electrical insulating properties.
It is still a further object of the present invention to provide a process for preparing a BN film having such excellent advantages as described above.
In accordance with the present invention, there is provided a boron nitride film grown on the surface of a substrate in a manner to be preferentially orientated to a predetermined axis thereof. In accordance with another aspect of the present invention, there is provided a process for preparing a boron nitride film comprising the steps of heating boride charged in a closed-type crucible having at least one injection nozzle to form boron vapor; ejecting said boron vapor through said nozzle into a nitrogen atmosphere having a pressure below 10.sup.-2 Torr and ionizing at least a part thereof; and impinging said ionized boron vapor on the surface of a substrate together with nitrogen to form a boron nitride film on said substrate.